A microelectronic module typically includes a semiconductor or other active device, electronic components, strip-line and/or wiring and interconnects, and a substrate that supports the foregoing elements. The substrate is a plate, wafer, panel or disk of suitable material on which (or in which) the components of an electronic unit, such as an integrated or printed circuit, semiconductor or other active device, electronic components, strip-line and interconnect wiring are deposited or formed. Accordingly, the material and shape of the substrate must possess physical and electrical properties suitable to the foregoing application. Typically, that material is a dielectric, an electrical insulator and the substrate is flat and relatively rigid.
Because of the desirable physical and electrical properties possessed by ceramic material, such as compositions of alumina (Al2O3), the ceramic is recognized as the material of choice for the substrate in microelectronic modules, providing the base on which to fabricate microelectronic semiconductors, printed wiring, electrical interconnects and the like. The ceramic material can be formed into the appropriate hard flat wafer or plate typically required of a module substrate.
The composition of a ceramic substrate for a microelectronic module application typically a percentage of alumina that may range between a maximum of just under 100% alumina and, at a minimum, 96% alumina. The remaining ingredients in the composition being a suitable binder that holds together the powder of the alumina and/or a combination of binder and a material proprietary to the manufacturer. Thus, in practice the alumina substrate never contains 100% alumina. Ceramic substrates are commercially available to the industry that are formed of compositions having a variety of standard percentages of alumina within the foregoing range of compositions. As example, 99.6% (by weight) alumina and 96% (by weight) alumina are two known commercially available compositions of ceramic substrate favored for microelectronic modules.
To manufacture the alumina substrate, alumina powder is physically broken apart by a ball mill to yield the desired size or range of sizes of particulate. Then the particulate is dispersed into a liquid forming a slurry. Particulate forms of glass, which serves as a high temperature binder, and polymer binders are added to complete the slurry. Small amounts of other, proprietary dielectric materials may be added to the slurry by the manufacturer. That slurry is adjusted to a viscosity that the manufacturer finds suitable to subsequent processing to form the substrate.
In one known process the slurry is evenly deposited on a flexible membrane, the carrier film, and formed into a film of constant thickness by applying (e.g. skiving) the slurry onto the surface of the carrier film using the edge of a flat blade, spreading the slurry evenly over the surface, a spreading procedure known as swiping, drying (e.g. firing) the slurry, followed by separating the dried slurry from the carrier film. When separated from the carrier film, the dried slurry forms a leather-like layer or film, referred to as “green tape.” In many operations, the carrier film comprises a moving belt that carries the slurry covered carrier film from the region of the blade through a dryer in a continuous process to produce the green tape.
The green tape can be cut, shaped or formed as desired. The green tape (or, as appropriate, a cut out or shaped portion of the green tape) is then placed in an oven or kiln and fired to temperatures in the range of 1600 Degrees Centigrade. At that high temperature, the glass (and any other component of the binder) melts or re-vitrifies, while the aluminum oxide remains a solid. Revitrification of the glass forms a matrix of the aluminum oxide and the binder. On cooling a hard rigid ceramic dielectric body of the desired shape for the substrate results.
A second known substrate manufacturing technique is to press glass and alumina powder mixture into a mold under high pressure to form a body to the desired shape. The inherent friction between the particulate of that mixture holds that body together initially. Thereafter, the formed body is fired as in the preceding process to form a bond of greater strength between the particulate material and produce the fused mixture.
The surface roughness of the fired substrate depends on the particle size of the alumina used to form the green tape. Some such substrate may be used “as fired” for subsequent processing that is tolerant of or requires a degree of surface roughness, such as in applications that use or apply thick films to the substrate. On the other hand, thin film applications are less tolerant of surface roughness due to the desired finish and the photolithographic processing employed to image the thin film on the substrate in lithographic processes. Most thin film applications thus require a substrate that has a polished surface. A sanding or grinding operation that uses diamond grit is used for that polishing.
Two familiar types of interconnects for a microelectronic module are RF interconnects and DC interconnects. The former is used to conduct RF energy from one location to another on a substrate, and the latter is used to conduct DC current from one location to another. The RF interconnect is typically in the form of a thin film, and the DC interconnect is, typically, a thick film. Frequently, both an RF interconnect and a DC interconnect are carried on a single substrate.
Thin film (eg. thin electrically conductive film) is applied to the substrate by metalization of the substrate surface, and is accomplished by a vapor deposition process. Typically titanium and tungsten are deposited in succession in 200 Angstrom thick layers, referred to as the bonding layer; depositing a layer of resistive material, such as tantalum nitride, to about a 200 Angstrom thickness; sputter depositing pure gold (eg. 99.99%) gold to a thickness of 10,000 Angstroms to form the “seed” layer, and then wet plating. The lines or pattern of electrically conductive lines, pads or regions are formed in the foregoing metallized layer or film using conventional photo-etch techniques, leaving other portions of the surface of the substrate exposed. Thereafter the formed lines can again be wet plated with gold by immersing the substrate in a wet plating bath, wherein the gold adheres to the conductive lines, but not to the substrate.
On the other hand, thick films are typically applied to a substrate using a screening process. In that process a photomask is applied to a very fine mesh polymeric or metallic screen and exposed in the negative image of the desired pattern. Then the unexposed portion is removed leaving the desired film pattern open. The electrically conductive film material is then squeegeed through the openings in the screen onto the surface of the substrate. Thereafter the substrate undergoes a series of high temperature treatments to solidify the thick film and the bond of the thick film to the surface of the substrate, the details of which are not material to an understanding of the invention. Thick film adheres poorly to a substrate. When a large substrate is subjected to a wide range of temperatures in application, the thick film may delaminate from the substrate.
Experience has taught that a substrate formed of a 99.6% (by weight) of alumina provides the best surface for the thin film (RF interconnect) processing, and that a substrate formed of a 96% (by weight) of alumina provides the best surface for the thick film (DC interconnect) processing. The foregoing difference in result is due in small part to the inherent surface finish of the two different compositions. For the most part the difference is believed due to the differences in reaction between the processes used to apply the materials that form the thick and thin films, respectively, to the ceramic substrate.
To the present, production engineers for microelectronic modules of the type that contain a substrate to support both thick and thin film conductors are forced to choose between a substrate composition whose physical characteristic better serves to bond to and support thin conductive film, that is, the 99.6% alumina composition, or bond to and support thick conductive film, that is, the 96% alumina composition. The compromise or choice in microelectronic module production is a ceramic substrate for the module that is optimal for application of thin conductive films (eg. the 99.6% alumina composition), notwithstanding the fact that the substrate must also bond to and support thick conductive films. As a consequence, de-lamination of the thick film from the good substrate occurs from time to time, decreasing the production yield of substrate or resulting in premature failure of the electronic module in which the substrate was applied.
The foregoing choice of ceramic substrate characteristic is due to the experience that an even lower production yield would occur if the physical characteristic of the substrate were to favor bonding of the thick conductive film. As an advantage, the present invention and method provides the appropriate physical characteristic to both the thin conductive film and the thick conductive film and, hence, enhances production yield of ceramic substrates and the reliability of the modules that use the substrate, even though the substrate is more complicated to manufacture than present ceramic substrates.
The new substrate retains the desired physical characteristics required for the additional aspects of substrate fabrication such as thick film filled communication vias, laser profiling for complex shapes and the inclusion of passive electrical elements formed of either or both thin conductive film and thick conductive film.
Accordingly, an object of the present invention is to improve of the production yield of ceramic substrates for microelectronic modules that incorporate both thick and thin conductive films and to increase the operational life of such microelectronic modules.
Another object of the invention is to produce a ceramic substrate that contains top and bottom surfaces of the same ceramic material but possess different physical characteristics, the physical characteristics of one being optimal for adherence of thick conductive film and the physical characteristics of the other being optimal for adherence of thin conductive film.
And still another object of the invention is to provide optimal adherence of the thick film conductors used for transmission of DC current and of the thin film conductors used for transmission of RF in substrates that support both such types of conductors concurrently.